This application is based on and incorporates herein by reference Japanese Patent Application No. 2002-151254 filed on May 24, 2002.
1. Field of the Invention
The present invention relates to an injection ratio control system for an internal combustion engine that drives injectors based on injection quantity command values such as an injector injection command pulse period, an injector energization period, an injector injection period, or an injector valve-opening period. The injection quantity command values are calculated from command injection quantity set in accordance with an operating state or operating conditions of the engine, and fuel injection pressure measured by a sensor and the like. Specifically, the present invention relates to a fuel injection system for an internal combustion engine capable of performing a multi-injection in which fuel is injected into cylinders of the engine in multiple times by driving electromagnetic valves of the injectors during a compression stroke or an expansion stroke of the engine.
2. Description of Related Art
Conventionally, as a fuel injection system for a diesel engine, an accumulation type fuel injection system is known. The accumulation type fuel injection system accumulates high-pressure fuel in a common rail as an accumulator and injects the high-pressure fuel to respective cylinders of the engine through electromagnetic fuel injection valves (injectors) respectively mounted in the cylinders of the engine. In the accumulation type fuel injection system, a command injection quantity is calculated in accordance with an engine rotation speed and an accelerator position. Command injection timing is calculated in accordance with the engine rotation speed and the command injection quantity. An injector injection period (an injection command pulse period) is calculated in accordance with an actual fuel pressure in the common rail, which is measured by a fuel pressure sensor and the like, and the command injection quantity. Then, an electromagnetic valve of the injector is applied with pulse-shaped injector driving current for opening a nozzle needle from the command injection timing until the injection command pulse period ends. Thus, the system controls the injection quantity and the injection timing of the fuel injected into the respective cylinders.
The accumulation type fuel injection system performs a multi-step injection (a multi-injection) in compliance with present restrictions on exhaust gas and noise. More specifically, the accumulation type fuel injection system performs the multi-injection to perform stable combustion from the start of a main injection so that noise and vibration of the engine are reduced and emission performance is improved. In the multi-injection, multiple pilot injections are performed before the main injection, which can generate engine torque. In each pilot injection, a small quantity of the fuel is injected. The electromagnetic valve of the injector of a specific cylinder is driven at least twice to open the nozzle needle of the injector at least twice during a compression stroke or an expansion stroke of the engine. Thus, the multi-injection including at least two injections is performed in the cylinder. For instance, the multi-injection includes one or more pilot injections and one main injection, or includes one pilot injection, one main injection and one after injection, or includes one main injection and one or more after injections. Thus, the system inhibits rapid increase in the injection ratio in an early stage of the combustion. As a result, the noise and vibration of the engine are inhibited.
In the conventional accumulation type fuel injection system, control of a minute quantity of the fuel is more important than ever when performing the above multi-injection. However, even if the minute injection quantity command value is calculated and outputted, the pilot injection may vanish or excessive fuel may be injected in the pilot injection due to variation in hardware. The variation in the hardware includes individual difference of the injector, variation in the injection quantities among the cylinders, or performance degradation of the injector with time. In such a case, noise reduction effect of the multi-injection may be reduced, and the actual injection quantity or injection frequency may fluctuate. As a result, fluctuation of drivability or emission may be caused.
It is therefore an object of the present invention to provide an injection ratio control system that regulates an injection frequency per combustion in a multi-injection if an injection quantity command value reaches a minimum guarantee injection quantity. Thus, the use of a minute injection quantity command value is prevented. As a result, variation in an actual injection quantity or an injection frequency is prevented. Thus, variation in the actual injection quantity or the injection frequency is prevented without reducing a noise reduction effect of the multi-injection.
According to an aspect of the present invention, a main injection quantity command value for a main injection in a multi-injection is calculated in accordance with an operating state or operating conditions of an internal combustion engine. An injection frequency per combustion in the multi-injection (a multi-injection frequency) is regulated so that the main injection quantity command value becomes equal to or greater than an N time(s) minimum guarantee determination value. Thus, vanish of a pilot injection or an excessive fuel injection in the pilot injection due to variation in hardware is prevented. As a result, the variation in the actual injection quantity or the injection frequency is prevented without reducing a noise reduction effect of the multi-injection.